Because gas is utilized only to a low degree in a fluidized bed reactor, the utilization of the gas usually must be improved by providing a plurality of fluidized bed reactors, namely, at least two and preferably three fluidized bed reactors, are connected in series. Alternatively a fluidized bed reactor can be used in which a plurality of fluidized beds are superposed and are traversed in succession as a result an improved utilization of the gas can be achieved in both cases in a counter-current operation.
The degree of utilization of the gas and the reducing rate is low at temperatures below about 600.degree. C. The fluidized bed is destroyed at temperatures above about 750.degree. C. because the reduced particles of iron ore then stick together so that they become too heavy for use in a fluidized bed. Besides, reduced particles of ore are preferentially sinter-bonded to metallic surfaces to form crusts, which more or less quickly restrict the flow through the fluidized bed reactor.
The reduction of iron ore is formally described by the following equation: EQU Fe.sub.2 O.sub.3 +3H.sub.2 .fwdarw.2 Fe+3 H.sub.2 O (1) EQU Fe.sub.2 O.sub.3 +3CO.fwdarw.2 Fe+3 CO.sub.2 ( 2)
Experience has shown that the degree of utilization of a gas in fluidized bed reactors is lower than in shaft furnaces, in which the transfer of mass and heat is optimized by the counter-current operation.
The freshly produced reducing gas (make-up gas) contains CO.sub.2 and H.sub.2 O as oxidizing components, particularly if it comes from a steam reformer.
In the FIOR process (The 38th Electric Furnace Conference, AIME, Dec. 9-12, 1980, Pittsburgh, Pa.), the gas coming from a steam reformer is initially fed to a CO shift converter and is then cooled to condense water and is passed through a scrubber to remove CO.sub.2.
Only after this expensive after-treatment does the freshly produced reducing gas have a sufficiently high reducing strength. But the thus purified gas must be reheated to the reducing temperature before it is fed as a fluidizing gas to the first fluidizing bed reactor of a multistage fluidized bed reactor plant, which consists of a plurality of fluidized bed reactors connected in series.